I think Saffron is a good fit - it sort of bridges your girls' names, which are quite elaborate and über-feminine, and your boys', which feel more modern, cool and tough. Saffron is a nice balance of the two styles but still sounds sweet and girly with Daisy in the middle.
If one name is also used as a regular word in English, then I think it really odd if the middle name is too. So for me that would rule out Daisy, Violet, Jade, etc. Otherwise it just invites the imagination to create a picture. Personally I think Saffron a bit to close in sound to Grayson but perhaps this is diluted by the various other siblings in between.